The separation of a gaseous mixture includes heat and mass transfer processes, thus strengthening the heat and mass transfer is very important for improving the separation effect of the gaseous mixture. For example, the cryogenic separation method is frequently used in petrochemical industry in order to separate desired ethylene and propylene products from the cracked gas. In this method the gaseous mixture is cooled or condensed at low temperature and under cryogenic condition by chilling system in series with adding refrigerant, partially liquidified and separated preliminarily into gas and liquid phases, and then the gaseous mixture is further separated via a series of processes such as rectification to yield products of high quality. With this method large amount of refrigeration will be consumed to obtain low temperature and low temperature resistant alloy is needed. In this regards, many improvements on the cryogenic separation process and its equipment have been made in recent ten years to decrease the amount of refrigeration and cut down the investment cost of low temperature equipment, among which one important improvement is employing a dephlegmator to condense a gaseous mixture for strengthening the heat and mass transfer. For example, U.S. Pat. No. 4,657,571 discloses a fractionation column 17 consisting of a dephlegmator 38; Japanese Patent Publication of Unexamination Application Nos. 5-87447, 6-337192 and 6-341760 disclose rectification columns consisting of dephlegmators. In the specifications of these patent and patent applications the details on the types of dephlegmators are not described, in the drawings they are shown as shell and tube types. For this shell and tube dephlegmator the heat and mass transfer effect is not very ideal due to the limited surface area, and it is not easy to control reflux during operation. Especially the operation load can not be very large, otherwise flooding will occur readily. Therefore how to improve the heat and mass transfer in a dephlegmator and increase the separation efficiency of the tower is of essential significance for enhancing the treatment capacity of the tower and lowering the cost.